1. Field of the Invention
The invention relates to the field of testing and diagnosing MOS transistors and MOS transistor fabrication and design.
2. Description of the Related Art
A direct-current current-voltage (DCIV) measurement technique of interface and oxide traps on oxidized silicon is demonstrated. The technique uses the gate-controlled parasitic bipolar junction transistor of a metal-oxide-silicon field-effect transistor in a p/n junction isolation well to monitor the change of the oxide and interface trap density. The debase and collector currents are the monitors, hence, this technique is more sensitive and reliable than the traditional ac methods for determination of fundamental kinetic rates and transistor degradation mechanisms, such as charge pumping.
It is well recognized that the electrical characteristics of metal-oxide-semiconductor transistors (MOST's) and bipolar junction transistors (BJT's) degrade during circuit operation due to channel-hot-electron (CHE) and substrate-hot-electron (SHE) stresses which increase oxide (Q.sub.OT) and interface trap (Q.sub.IT) densities [1], [2].sup.1. In MOST's , the trapped charges reduce the mobility (.DELTA..mu.) and shift the threshold gate voltage (.DELTA.V.sub.GT), both of which reduce drain saturation current (.DELTA.I.sub.D) which slows down the switching speed due to longer charging time of interconnect or load capacitances at lower currents. The trapped charges also shift the subthreshold gate voltage (.DELTA.V.sub.GT-sub), and decrease subthreshold slope of the drain-current versus gate-voltage curve, which reduces the current cut-off sharpness, thereby increasing leakage current or standby power and decreasing the noise margin. In BJT's , Q.sub.IT and Q.sub.OT will increase the minority carrier recombination rate in the base, thereby reducing its current gain, such as the common-emitter current gain, .beta..sub.F [3]. Thus, a quantative separation of the effects of Q.sub.OT and Q.sub.IT is necessary to delineate the location and physical origin of the degradation in order to design and manufacture highly reliable integrated circuits with ten-year or longer operating life.
The separation of Q.sub.OT and Q.sub.IT is generally difficult. It has not been reliably separated using the traditional capacitance and conductance methods or the transient methods because the test structures are two-terminal capacitors, or very small test transistors which give extremely small capacitances due to the very small device area. Many traditional methods for separating Q.sub.OT and Q.sub.IT were reviewed [4], and a two-step method was demonstrated. However, it uses the subthreshold slope to monitor Q.sub.IT which is reliable only when there is not an inhomogeneous or lateral distribution of Q.sub.IT and Q.sub.OT. Hence, it is not reliable for monitoring the highly nonuniform Q.sub.IT and Q.sub.OT generated by CHE stress.
A novel method is demonstrated in this paper which measures the de base and collector currents versus the gate voltage, to be known as DCIV method (in analogy to the traditional usage such as HFCV for high-frequency capacitance-voltage or QSCV for quasi-static CV), to monitor the Q.sub.IT and Q.sub.OT. The novel DCIV method contains two features: 1) The base current (I.sub.B) of the vertical BJT is used to measure the recombination current at the interface traps generated during fabrication or operation which avoids the error from lateral distribution or areal nonuniformity of Q.sub.IT and Q.sub.OT because I.sub.B is directly proportional to N.sub.IT or Q.sub.IT /q. 2) The Collector current (I.sub.C) of the vertical BJT is used to measure the Q.sub.OT because I.sub.C increases sharply when the gate voltage passes the flat-band value toward depletion and inversion. The method will be described in this article using the nMOST and npnBJT of the BiMOS structure shown in FIG. 1. This BiMOS structure has been used previously to fabricate large test transistors wish nearly 400,000 .mu.m.sup.2 gate oxide area by Thompson ([8] and [9] cited in [4]), but it is also present in the submicrometer nMOST's in a p-well on n-substrate of production CMOS (Complementary MOS) inverter circuits. Thus, the novel DCIV method to be described can be easily applied to production test transistors and some examples to be given were data measured on micrometer and submicrometer MOST-BIT production structures.
With reference to FIG. 1, the BIT can be measured before and after a stress in two configurations: The top-emitter (top-E) or bottom-emitter (bottom-E) measurement configurations, with the n+drain/p-base or n+substrate/n-epitaxy/p-base as the forward biased emitter/base junction. Our geometrical terminology deviates from the traditional, emitter-up and emitter-down, which confuses the geometrical location of the emitter with the emitted-charge flow direction. In both configurations, the shape of the I.sub.B -V.sub.GB curve and the magnitude of I.sub.B at a constant V.sub.EB will measure Q.sub.IT [5], [6]. However, we recently anticipated that the shape and magnitude of I.sub.C will also be a strong function of V.sub.GB in both configurations because I.sub.C increases sharply at the flat-band gate voltage, V.sub.GB-flatband, from a low constant current t a high constant current at strong inversion voltage V.sub.GB-threshold. This sharp increase occurs when the electron-channel between the n+drain and n+drain and n+source appears at V.sub.GB-flatband which abruptly increases the emitter/base area in the top-E configuration and the collector/base area in the bottom-E configuration.
The stress-induced base current, .DELTA.I.sub.B, is solely due to electron-hole recombination at the stress-generated interface traps [5], [6], hence, is a function of stress-induced interface charge and trap concentrations, .DELTA.Q.sub.IT and .DELTA.N.sub.IT, to the stress-induced density-of-states of the interface traps and surface recombination velocity, .DELTA.D.sub.IT and .DELTA.S.sub.Q. However, the increase of the collector current with V.sub.GB is nearly all from geometrical increase in the emitter or collector area contributed from the nMOST's electron channel. Therefore; the lateral shift in the I.sub.C -V.sub.GB curve, .DELTA.V-GB, is mainly a function of the stress-induced change of flatband gate voltage, .DELTA.V.sub.GB-flatband+ and hence is a very sensitive monitor of .DELTA.Q.sub.QT +.DELTA.Q.sub.IT. Thus, combining the .DELTA.I.sub.B -V.sub.GB and I.sub.C -V.sub.GB data will enable the separation of .DELTA.Q.sub.OT and .DELTA.Q.sub.IT. Experimental data in the following section will demonstrate this capability of the novel DCIV method.
Minority carrier surface recombination rate or velocity S.sub.o at the Si/SiQ.sub.2 interface was studied extensively since the use of MOS-gate-controlled BJT was demonstrated by one of us in 1961-1962 [5], [6]. In the early and follow-up experiments, I.sub.B was measured in either the top-emitter configuration [5]-[9] or bottom-emitter configuration [10]-[13], to evaluate S.sub.O. In [7] through [9], the BJT .beta..sub.F degradation during emitter-base reverse-bias stress at the junction breakdown voltage was also studied. In many of these earlier measurements, the I.sub.B -V.sub.GB curve was also displaced along the gate-voltage axis due to stress, but the peak in I.sub.B -V.sub.GB was not very sharp. In some cases no peak was observed. In addition, the magnitude of I.sub.B was greatly increased by the generated N.sub.IT. Thus, an estimate of .DELTA.Q.sub.OT from the shift of V.sub.GB at the peak .DELTA.I.sub.B in the .DELTA.I.sub.B -V.sub.GB curve cannot be very accurate and reliable.
Production n-channel MOST fabricated by state-of-the-art CMOS process is measured to demonstrate the proposed DCIV method. The starting n-Si wafer has a p-base well with surface concentration of 1.times.10.sup.16 cm.sup.-3, gate oxide thickness of x.sub.O.congruent.150 .ANG.. channel length L=1.6 .mu.m, and the gate area of A.sub.G =1.6.times.100 .mu.m.sup.2. The cross-sectional view was shown in FIG. 1.
FIG. 2(a) and (b) shows the npn-BJT's I.sub.B -V.sub.GB and I.sub.C -V.sub.GB curves, measured in both the top-E and bottom-E configurations, before and after SHE stress as labeled. The oxide charges and interface traps were generated by areally uniform SHE stress with V.sub.SB =V.sub.DB =4 V, and V.sub.GB =7.5 V. During SHE stress, the bottom emitter junction (n+substrate/n-epitaxy/p-base shown in FIG. 1) was forward-biased to inject electrons into the p-base. Some of these electrons are accelerated (designated as hot electrons), by the reverse-biased surface space-charge layer (V.sub.SB -V.sub.DB =4V) of gate-induced collector/base junction area, to &gt;3.2 eV kinetic energy. These hot electrons are then injected into the gate oxide over the 3.12 eV SiO.sub.2 /Si electron potential barrier. Some of the injected electrons are captured by the neutral oxygen
vacancy centers [14]-[15], giving V.sub.O +e.sup.-.fwdarw.V.sub.O.sup.- and the negative Q.sub.OT or positive .DELTA.V.sub.GT. Because of their high kinetic energy (.about.4 eV from V.sub.DB =V.sub.SB =4V) which is greater than the bond energy (.about.3 eV) of the strained Si--Si and Si--O interfacial bonds and the interfacial Si--H and Si--O bonds, the hot electrons also created some new interface traps, N.sub.IT or D.sub.IT, as indicated by the large increase of I.sub.B in FIG. 2(a) measure in both the top-emitter and bottom-emitter configurations. The build-up of Q.sub.IT also decreases the subthreshold slope of the nMOST's I.sub.D -V.sub.GB curve shown in FIG. 2(c), however, the V.sub.GB shift in I.sub.D-V.sub.GB is due to the build-up of both Q.sub.OT and Q.sub.IT ##EQU1##
which cannot be separated by this MOST I.sub.D -V.sub.GB measurement alone unless additional properties of the interface traps are known or assumed, a limitation also present in Terman's method to obtain D.sub.IT from HFCV characteristics. The two BJT measurements just described in FIG. 2(a) and (b) can help to separate the Q.sub.OT and Q.sub.IT, which are analyzed as follows.
The stress-generated increase of the I.sub.B shown in FIG. 2(a) gives a direct measure of the surface recombination velocity S.sub.O and the density-of-the-state of the interface traps, D.sub.IT because it is proportional to the maximum of the stress induced I.sub.B.DELTA.I.sub.B.ident.I.sub.B (post-stress) -I.sub.B (pre-stress). Its peak and shape can be distorted by areal nonuniformity of Q.sub.OT, D.sub.IT, or other device parameters, such as base dopant concentration and oxide thickness. But areal nonuniformity alone cannot produce a base current which must come from electron-hole recombination, unlike the HFCV (C.sub.gb -V.sub.GB) used in Terman's analysis and the I.sub.D -V.sub.GB in the subthreshold slope analysis of the interface trap density, whose distortion could solely arise from areal inhomogeneity even when D.sub.IT =0.
The V.sub.GB-IT component can then be calculated from (2B) given below [5] EQU .DELTA.I.sub.B.congruent.(qA.sub.G n.sub.i.DELTA.S.sub.O /2)exp (qV.sub.BE /2kT) (2) EQU .DELTA.S.sub.O.congruent.(.pi./2).tau..sub.O (-).sub.th.cndot..DELTA.N.sub.IT (2a) EQU .DELTA.Q.sub.IT.ident.-C.sub.O.DELTA.V.sub.GB-IT.ident.q.DELTA.N.sub. IT.congruent.q.DELTA.D.sub.IT.cndot..DELTA.E.sub.IT (2b)
The density-of-state, .DELTA.D.sub.IT (1/cm.sup.2 -eV), and carrier capture cross sections, .tau..sub.n =.tau..sub.p =.tau..sub.O (cm.sup.2), of the interface traps are assumed to be independent of the binding energy in the energy range .DELTA.E.sub.IT in the Si energy gap [16]. The calculation of .DELTA.V.sub.GB-IT is more complex than (2)-(2B) for an energy distribution of interface traps with energy-dependent density-of-state, D.sub.IT (E.sub.IT) and carrier capture cross sections, .tau..sub.n (E.sub.IT) and .tau..sub.p (E.sub.IT). However, S.sub.O calculated from measured .DELTA.I.sub.B after stress using (2), can still be used to monitor the build-up of the interface traps and the associated .DELTA.V.sub.GB-IT. In the example shown in FIG. 2(a), the numerical results are .DELTA.S.sub.O.congruent.1600 cm/s at the I.sub.B-pak which occurs at the gap energy position of V.sub.S -V.sub.F =-0.24 V below the midgap for the top-E curve stressed with a fluence of 5.times.10.sup.18 electron/cm.sup.2, and .DELTA.S.sub.O.congruent.40 cm/s at V.sub.S -V.sub.F =-0.26 V for the bottom-E curve stressed at a fluence of 1.times.10.sup.17 electron/cm.sup.2. For many devices measured, I.sub.B-peak of the bottom-E was about five times smaller than that of top-E.
The prestress-poststress I.sub.C -V.sub.GB curves of both the top-E and bottom-E configurations shown in FIG. 2(b) give a very sensitive measure of the stress-generated V.sub.GB shift. I.sub.C is flat in the accumulation range and is proportional to the area of the n+drain/p-base well junction (or the sum of the area of n+drain and n+source if drain and source are tied together during the I.sub.C measurement). When V.sub.GB.gtoreq.V.sub.FB.congruent.-0.55 V (Greater sign is for nMOST), an electron surface channel begins to form which will collect the electrons injected by the bottom-emitter and pass the collected electrons to the n+drain or/and n+source, causing an increase of I.sub.C (I.sub.D +I.sub.S). The I.sub.C quickly reaches a high plateau as V.sub.GB increases further to about -0.15V. This increase of I.sub.C is proportional to the added collector area from the gate-induced electron-channel. The three characteristic Si surface potentials of Si energy band bendings (FB-flatband at V.sub.S -0 V, INV=inversion at equal electron-hold surface concentration N.sub.S =P.sub.S or V.sub.S =V.sub.F -V.sub.BE /2, and TH=threshold or strong inversion at N.sub.S =P.sub.base or V.sub.S =2V.sub.F -V.sub.BE) are marked by dots on the pre-stress I.sub.C -V.sub.GB curve in FIG. 2(b). They show that I.sub.C starts to rise sharply at V.sub.FB.congruent.-0.55 V at flatband in this example, reaching the higher plateau about halfway between inversion V.sub.GB-inversion.congruent.-0.25 V and the MOST threshold voltage, V.sub.GB-th.congruent.+0.05 V. Thus, the rise of I.sub.C is sharp and occurs in a short range of V.sub.GB, in this case, -0.05 V-(-0.55 V)=0.5 V.
It was asserted in the preceding discussion that I.sub.C is not caused by carrier recombination or generation of the newly generated interface traps, but solely by the increase of the emitter or collector area from the gate-induced electron channel described above. This is not experimentally proven in FIG. 2(b) by the nearly parallel V.sub.GB shift of the post-stress I.sub.C from its-pre-stress range with the nearly identical height for both the top-E and bottom-E measurement configurations, although this stress has generated a large N.sub.IT to give the large increase of I.sub.B shown in FIG. 2(a). This model is further supported by the observed and anticipated reduction of slope of the post-stress I.sub.C -V.sub.GB at higher V.sub.EB bias reflecting a larger negative Q.sub.IT (.congruent..DELTA.D.sub.IT.cndot..DELTA.E.sub.IT). This is expected from the higher surface electron concentration injected by the emitter to charge the interface traps negatively due to i) the added stress-induced .DELTA.D.sub.IT, and ii) a larger energy range of N.sub.IT towards the Si conduction band edge, estimated by .DELTA.E.sub.T.about.V.sub.BE.
A quantitative analysis of the Q.sub.IT contribution to I.sub.C -V.sub.CB shift in FIG. 2(b) can be made from EQU .DELTA.V.sub.GB-IT =-.DELTA.Q.sub.it (V.sub.F -V.sub.FN)/C.sub.O (3)
by using the fundamental property of intrinsic interface traps whose charge state is acceptor-like (negatively charged) near the conduction band edge and donor-like (Positively charged) near the valence band edge, because they are localized or bound electron states which are split-off states from the respective band states by random atomic location perturbation of the crystalline periodic potential at the SiO.sub.2 /Si interface. This charge state assignment was implied by Bardeen when he introduced the concept of neutral Fermi level V.sub.FN [17]. For Si, V.sub.FN is at about E.sub.V +(1/3)E.sub.G [17], [18]. Thus, V.sub.F -V.sub.FN.congruent.O at flat-band for the .sub.p --Si used here which was doped with N.sub.AA.about.10.sup.16 cm.sup.-3 of boron acceptors, and C.sub.o.DELTA.V.sub.GB.congruent.-.DELTA.Q.sub.QT. The stress induced V.sub.GB-FB shift in the I.sub.C -V.sub.CB curves of FIG. 2(b) then gives EQU .DELTA.N.sub.OT =(C.sub.o /.sub.q).DELTA.V.sub.GB-OT (4) EQU .congruent.(C.sub.o /q)[-0.2-(-0.55)]=4.3.times.10.sup.11 cm.sup.-2 (4a)
which can then be used in (1) to separate Q.sub.IT and Q.sub.QT. In view of the nearly parallel shift of the I.sub.C -V.sub.GB curves at low V.sub.BE (.about.0.3 V in these examples), it is not necessary to locate the V.sub.GB-FB point to get .DELTA.V.sub.GB-OT point to get .DELTA.V.sub.GB-OT in practical applications. The change of the subthreshold slope of the MOST I.sub.D -V.sub.GB slope has been commonly used to monitor Q.sub.it. It is accurate if N.sub.it is areally constant. This is untenable in the practical CHE stress and can give erroneous results [4], [19]..sup.2 In the present example, N.sub.it was generated by the areally uniform SHEi stress, thus, a decreasing slope of I.sub.D -V.sub.GB shown in FIG. 2(c) gives an indication of a real stress-induced N.sub.IT rather than inhomogeneity, analogous to the reasoning for the anticipated experimental post-stress slope reduction of the I.sub.C -V.sub.GB just discussed. However, an important point is frequently overlooked: N.sub.it or D.sub.IT (E.sub.IT).DELTA.E.sub.it from the I.sub.P -V.sub.GB in FIG. 2(c) is in the strong inversion voltage range, V.sub.GB &gt;V.sub.CB-TH or in the Si-gap energy range V.sub.S &gt;2V.sub.F -V.sub.BE (2kT/q) near the conduction band edge. In contrast, N.sub.IT or D.sub.it (E.sub.it).DELTA.E.sub.it in FIG. 2(b) is in the mid-range of the Si energy gap from flat-band (V.sub.s =0) to strong inversion or threshold (V.sub.S =2V.sub.F -V.sub.BE). Thus, the subthreshold slope monitors an additional energy range of D.sub.IT near the minority band edge, which is application-important because the MOST operates in this strong inversion range when it is turned on. But it is also fundamentally significant because the decreasing post-stress subthreshold slope [compressed by logarithmic I.sub.d but still visible in the solid curve FIG. 2(e)] indicates an increasing D.sub.IT with energy towards the conduction band edge, which is consistent with the commonly depicted U-shaped D.sub.IT as anticipated by the fundamental microscopic-atomic model of interface states implied by Bardeen [17]. A qualitative estimate from the subthreshold slope change in FIG. 2(c), using the well known equation [20] gives EQU D.sub.IT =(C.sub.O /.sub.q)(.sub.q /2.303 kT).DELTA.S EQU .congruent.10.sup.12 cm.sup.-2.sub.c V.sup.-1
Additional examples are given in FIG. 3(a)-(e) for the bottom-Emitter configuration which use I.sub.C to monitor negative, positive, and turn-around .DELTA.Q.sub.QT induced by stress. FIG. 3(a) is identical to FIG. 2(b) showing positive .DELTA.V.sub.CB from negative .DELTA.Q.sub.GB after SHEi stress (curve 2) with V.sub.GB =12 V and V.sub.DB =V.sub.SB =10 V, due to positive .DELTA.Q.sub.QT, as anticipated [15] by the electron-impact emission of electrons trapped at the neutral oxygen vacancy, V.sup.( ) +c.sup.o.fwdarw.V.sub.( ) +2e-. FIG. 3(c) demonstrates the successive stresses that gave negative .DELTA.Q.sub.QT first and then positive .DELTA.Q.sub.QT, which is the so-called turn-around effect coined by Young [21]. Curve 1, showing positive .DELTA.V.sub.GB, was measured after a short (.about.1 s) CHEi stress at V.sub.GB =V.sub.DB.congruent.15 V with the source floating, indicating negative .DELTA.Q.sub.QT due to capture of the electrons injected into the oxide along the entire length of the strongly inverted n-channel because V.sub.GB &gt;&gt;V.sub.TH. Curve 2, showing negative .DELTA.V.sub.GB, was measured after an additional 500 s stress, indicating that some originally trapped electrons (not the captured electrons during the short stress) are emitted via a second pathway, the impact emission just described for FIG. 3(b).
The stress condition used in FIG. 3(c), with source floating or shorted to drain, approximates that in BJT under emitter-base junction reverse-bias stress. Thus, the bottom-emitter measurement configuration can be used to study the fundamental degradation mechanisms in BJT even without a separated gate over the emitter-base junction [3].
The sensitivity of this new DCIV is demonstrated experimentally in FIG. 4 which gives a sensitivity limit or minimum measurable S.sub.O &lt;.about.1 cm/s and N.sub.IT.ltoreq.10.sup.9 cm.sup.-2.